The Iraqi journalist who threw his shoes at George Bush was tortured into writing a letter of apology, his brother said today.

Muntazer al-Zaidi was wrestled to the ground after throwing his shoes during a news conference held by the US president and the Iraqi prime minister, Nouri al-Maliki, on 14 December.

The investigating judge in the case said last week that Zaidi, who will stand trial on 31 December, was beaten around the face and eyes. Zaidi's brother, Uday, said the journalist suffered worse injuries, including a missing tooth and cigarette burns to his ears, and would sue.

Zaidi is in custody and faces up to two years in jail for insulting a foreign leader. Maliki's office said last week that Zaidi had written a letter of apology and asked him to recommend a pardon.

Uday al-Zaidi said his brother told him the letter was written against his will because he was subjected to torture, including being doused with cold water while naked.

"He told me that he has no regret because of what he did and that he would do it again," Uday said. "The thing that makes you cry and laugh at the same time is that when the prime minister said that my brother was not tortured and will not be tortured, he was under severe torture by security authorities."

Maliki said Zaidi admitted in the letter that a terrorist had induced him to throw the shoes. "He revealed … that a person provoked him to commit this act and that person is known to us for slitting throats," Maliki said, according to the prime minister's website. The alleged instigator was not named.

Maliki said his government remained "committed to protecting the journalist in performing his professional duty," and guaranteed him the right to practise his profession "on condition that he does not violate the dignity of others".

Neither Bush nor Maliki have sought charges, but investigating judge Dhia al-Kinani said last week he did not have the legal option to drop the case.

Footage of Zaidi attacking Bush was repeatedly broadcast worldwide and he has become a symbol for opponents of the US-led invasion and occupation of Iraq. Thousands of Iraqis have rallied to demand his release.